1. Field of the Invention
The invention relates to a radio FM receiver provided with a voltage-controlled oscillator which can be connected via a frequency 2-divider to a quadrature combination circuit for transforming down the frequency of received radio signals and for supplying quadrature output signals, as well as with detection and sound reproduction means connected to said combination circuit.
2. Description of the Related Art
A synthesizer is used in electronically tuned radios for tuning a voltage-controlled oscillator (VCO) to a desired reception channel. In practice, the VCO signal is directly supplied to a buffer and subsequently to a combination circuit. In a receiver architecture with integrated means for the suppression of image frequencies, however, the VCO is often tuned to a frequency higher than the frequency necessary for tuning to the desired channel. In such a situation, the oscillator is followed by a frequency divider, in particular a frequency 2-divider, for serving a quadrature combination circuit which renders possible a suppression of image frequencies. The output signals thereof may then be supplied to two buffers and subsequently, on a quadrature basis, to combination circuits connected thereto and operating at the same frequency.
Such a radio FM receiver is known, for example, from U.S. Pat. No. 5,761,615.
A car radio IC with the type designation TEA 6840 was developed in recent years. An object in the further development of this IC is to enhance its functions by providing a possibility of replacing blocks or adding functions in a manner such that the further development has a minimum influence on the basic construction of the IC.
The reception of the so-called Japanese band is one of the respects in which the TEA 6840 IC, which is attuned to the European/American band, is in need of improvement. A change in the tuning part is required for enhancing its functions. As was noted above, such a change must not result in a change in the basic construction of the IC; the tuning range of the VCO frequency must remain unchanged.
The desired reception channels in the FM mode usual in Europe and America lie between 87.3 MHz and 108.3 MHz. In Japan, however, the desired reception channels lie between 76 MHz and 90 MHz. Table I below lists the frequency bands of the reception channels, the oscillator frequency ranges, the combination frequency, the obtained center frequency, and the tuning voltage range of the oscillator for the situation in Europe/America (EU/US), a situation in Japan with the use of a 2-divider (2-JP), and the traditional situation in Japan (1-JP).
The frequency 2-divider supplies a signal with a combination frequency fEU in a range from 98 MHz to 119 MHz in the case of an oscillator frequency fVCO lying in the range of 196 MHz to 238 MHz. The center frequency obtained in the mixer element during tuning for the European/American band, then lies at 10.7 MHz. The tuning voltage for the oscillator may vary from approximately 2 to 5.5 V in that case. For the Japanese band (from 76 to 90 MHz; see 2-JP in Table I), the same frequency 2-divider could be used for obtaining the center frequency of 10.7 MHz with an oscillator frequency in the range from 173.4 to 201.4 MHz. The tuning voltage for the oscillator may then vary from approximately 1 to 2.5 V. This situation is undesirable, however, on account of tuning problems which occur in practice. In the situations mentioned above, the frequency to which the unit is tuned lies below the combination frequency. In the traditional situation in Japan (1-JP in Table I), however, the combination frequency lies below the frequency to which the unit is tuned. Image frequency suppression problems then arise in the manufacture of a universal receiver which is suitable both for the European/American band and for the Japanese band.
It is, accordingly, an object of the invention to provide a radio FM receiver which is suitable for radio reception in Europe and America as well as in Japan and in which the above problems are avoided.
According to the invention, the radio FM receiver as described in the opening paragraph is, for this purpose, characterized in that, in addition, a frequency 3-divider is present by which the oscillator can be connected to the combination circuit, as well as switching means for connecting the mixer element to the oscillator either by the 2-divider or by the 3-divider.
Given an oscillator frequency fVCO lying, for example, in the range from 196 MHz to 238 MHz, the 2-divider will supply a signal with a combination frequency fEU in the range from 98 MHz to 119 MHz, and the 3-divider will supply a signal with a combination frequency fJP in the range from 65.3 MHz to 79.3 MHz. The center frequency obtained in the mixer element during tuning will then lie at 10.7 MHz both for the European/American band and for the Japanese band. The measures according to the invention render it possible to avoid the problems arising from the difference in frequency bands for Europe/America and Japan and the accompanying image frequency suppression problems, as well as the differences in VCO tuning voltages; see Table II
If a frequency 2-divider only is used, i.e., if the European/American and Japanese frequency bands need not be combined, the quadrature signals may easily be obtained with a duty cycle of 50% through switching on the flanks of the VCO signal. A frequency 3-divider with a duty cycle of 50%, however, cannot be obtained in this manner. If the flanks of one of the quadrature signals coincide with the flanks of the VCO signal, the flanks of the other quadrature signal will coincide with the center of a pulse in the VCO signal; the latter quadrature signal cannot be obtained then through switching on the flanks of the VCO signal. It is obviously also possible to generate the second quadrature signal in that the first one is subjected to a phase shift of 90xc2x0. This, however, will lead to errors of the order of xc2x13xc2x0 for the frequency band under discussion here, which is unacceptably high.
To solve this problem, the radio FM receiver is, in addition, characterized in that the frequency 3-divider is provided with a divider circuit which supplies n cyclical output signals Vn(t) for every three VCO signal cycles, for which it is true that Vn(t)=V1(txe2x88x92(nxe2x88x921)xcex94t) with n=1, . . . , 6, and xcex94t=the duration of half a VCO cycle, and is provided with two summation networks in each of which the cyclical output signals are combined with predetermined weighting factors designed for obtaining a quadrature component, whereupon the combination signal thus obtained is conducted through a low-pass filter. A frequency 3-divider which is highly accurate in generating quadrature signals becomes possible in this manner, which implies that it also makes sense to include it together with a 2-divider in the radio FM receiver so as to make the latter suitable for reception both in the European/American band and in the Japanese band.
In a preferred embodiment, the weighting factors for obtaining a first quadrature component have a mutual ratio of (xe2x88x922, +2, +4, +2, xe2x88x922, xe2x88x924), and those for obtaining the other quadrature component have a mutual ratio of (xe2x88x923, xe2x88x923, 0, +3, +3, 0). Cyclical interchange is to be included therein.
The invention relates not only to a radio FM receiver, but also to a frequency divider which can be used in a radio FM receiver. According to the invention, said frequency divider is provided with a divider circuit which supplies n cyclical output signals Vn(t) for every 1/2n oscillator signal cycles, for which it is true that Vn(t)=V1(txe2x88x92(nxe2x88x921)xcex94t), with n being an integer number and xcex94t the duration of half an oscillator cycle, and is provided with at least one summation network in which the cyclical output signals are combined with predetermined weighting factors, whereupon the combination signal thus obtained is conducted through a low-pass filter. A comparatively simple 3-divider for use in a radio FM receiver as described above is obtained for n=6 and with two summation networks. Alternative values of n are also possible. n may be equal to 4 in the case of a 2-divider. It is also possible to form signals with a certain frequency from any number of oscillator cycles through adaptation of the weighting factors; a direct relation between the number of oscillator cycles and the frequencies to be formed is not necessary in that case.